I have been thinking about making a tube preamp for my guitar. The tube pre is supposed to color the sound a bit, and/or distorting (clipping) before it enters my soundcard/or maybe a guitar amplifier. I have been reading "Valve Amplifiers" (third edition) by Morgan Jones (I'm not fully through it). A guitar preamp will work with very small signals (typically around 250mV RMS), and will not need to amplify the signal, but the possibility of 2:1 or maybe 3:1 would be a great bonus (to get slightly more gain in my soundcard). It does not have to amplify up to line level.

According to the theory in the book, I would need a very high power supply for this. Judging from the curves on the typical triode spec sheets, anode voltages down to 9V (typical battery power) should be downright impossible (Vgk would be close to zero). Yet, the reputed amplifier giant Vox sells battery powered preamps, that runs for 16 hours on 4XAA batteries:

The tubes are 12AU7, and seems to be an old type, not specialised for low voltages. They explain it is a patented circuit, and if it is patented, they are required to make the theory/knowledge behind the invention public. So I did a search among US patents and found several circuits that run conventional tubes on batteries, but it's hard to know whether it's the same technique, or other techniques (they don't tell what company holds the rights to the inventions). I found one from as early as 1991, so the technique has been known for long, but it's very long winded, and they seem rather vague about what actually makes it work at such low voltages, so I haven't gotten the time to figure out what the clue is. Searches on battery powered tube devices on guitar boards reveals almost everyone thinks it's scam, or produces low quality sound, yet the effects pedals from Vox receive great reviews from musicians.

Soo.... here's my questions:
1) Is this worth looking into (=can it produce good quality, will it be lower quality than at full voltages etc...)

if yes...

2) Where can I find information about theory etc. so I can make one myself?

Can you tell me anything more about this?

[BTW I know Vox are from UK not US, but didn't know where to search UK patents, and don't know where it may be registered]

There is a guy on ebay who sells a ready built pcb, item number 260385003973.
I have bought one of these for a nixie clock. It worked out easier to just buy the little ready built pcb than hunt about for the Maxim chip.
If you stick a couple of silicon diodes in the ground lead of the 5V reg and heatsink it, then thats your heater supply.
It will eat batterys though.

From what I have read, the “tube” guitar pedals/gizmos that use 9vdc as a power supply input are not using the tube as an amplifier. Rather the tube is in “diode mode” and is being used to clip off some of the signal to get distortion. No "signal" is going thru the tube. So it is “tube distortion” (for legal/marketing purposes) but not in the sense that most people would expect.

The only devices I'm aware of that use tubes configured as clipping diodes are Fenders Roc Pro -series amplifiers. They have patented the circuit.

Vox Cooltron pedals feature this tube stage:

Aside what seems to be a DC bias servo, it's a rather normal gain stage. It has very "hard" clipping characteristics. You won't find that hyped "soft tube saturation" from this design. Most of the magic of those pedals lies in the other parts of the circuitry (solid-state diode clipping gain stages, frequency response shaping etc.).

Originally posted by pbholmen
[B]
The tubes are 12AU7, and seems to be an old type, not specialised for low voltages.

There were a few valve types designed for low voltage use, and the 12U7 was one of them. However, the 12AU7 seems to be more-or-less identical! -It also works ok at low voltages. The 12AT7 does too, but the 12AX7, forget it.

The problem with all these circuits is the heater. You need hardly any current for the anode -no problem- but you still need a lot of power for the heaters, which drains your batteries pretty quick. Some have tried running the heaters at reduced current, but tone always seems to suffer (I think Takamine make a valve buffer for acoustic guitar which runs the heater at 1.5V! It sounds awful).

I have great sucess building valve buffer and treble booster pedals running off 9-12V (ECC82 and ECC81), but I always use a mains adapter (wall wart).

One thing which has crossed my mind for low voltage applications is usinge a vacuum flourescent display, which is bascially a triode...

Thanks for a lot of interesting replies. I have done some research, and found:

1) The patent that I found from 1991, which is very detailed, turned out to be a distortion pedal which receives rave reviews, have been called The Holy Grail of distortion pedals, and have been used by David Gilmour, Joe Satriani, Eric Johnson etc... and uses 12AX7-A. It seems, according to the patent, the secret is not about the type of tubes, but circuit topology (correct me if I'm wrong here). Many tubes in the 12A*7 series have been used for these devices.

2) The only solution I found which ran on batteries, as opposed to DC battery eliminator, was the Vox Cooltron series, exactly for the reasons Merlinb mentions (heater current would drain batteries). Vox claims to have a patent that solves this, but it's not essential to run it on batteries. Just lowering the voltage required to around 9-12V would have tons of obvious advantages.

3) The vacuum tubes ARE in the signal path in these devices, and DO amplify.

4) Looking at the circuit in the patent from 1991 (BK Butler Tube Driver), it looks like linearity would be poor in the tube amplification part. This would be an issue for HiFi-purposes, and may be a reason Morgan Jones doesn't mention these circuits. Distortion in guitar preamps can be a good thing (even at clean settings), because the signal from a solidbody guitar is very dead, and needs colouring anyway.

The reason I thought it would not be possible, is that Morgan Jones said one must avoid grid voltages near or above -1 volts. Then grid current starts to flow, and the resistance between grid and cathode can no longer be taken to be infinite. Judging from 12AU7 specs, both increased grid voltage, and lowered anode voltage seems to make the grid resistance lower when Vgk is above 0v. From most triode spec sheets, it looks like grid voltages below -1 volts would completely shut the current (Ia) down with such low anode voltages.

Judging from the circuit on page 4, Fig 4B, he keeps the grid voltage positive, and grid current will attenuate the signal. My guess is that it will be unevenly attenuated, and therefore not linear, which is quite alright really, as long as it's coloured a nice musical way. Anyway I'll try to build this one, it costs $300 plus shipping from their website in the USA, and doesn't come with powerplug for my country (plus I'd have to add 25% VAT to get it)...

It turns out the schematics in the patent are quite different from the finished product. A guy (Bajaman) at freestompboxes.org has traced and made schematics for the original BK Butler Tube Driver, that's the one that received such rave reviews. Many people have tried remakes of the circuit in the patent, and say it doesn't sound so good. Here it is:

This one uses an internal power adapter, which adapts to 12V AC, and then steps up to 24V DC. Bajaman also have made an improved circuit, you can find links to it in the same thread (page 3). The improved circuit uses external power adapter, thank god, thought it's 16V AC which isn't easy to find, and steps it up similarly, (to around 36V DC I guess... haven't looked at the circuit properly yet)

If you are concerned about the low B+ voltage you can always use a voltage multiplier -type power supply. A DC power source, such as a battery, will naturally need an oscillator in order to get the multiplier working.